1. Field of the Invention
The present invention relates to a video camera, and more particularly, to an improved black level calibration apparatus for a video camera which is capable of automatically calibrating a black level of an applied image signal inside a chip.
2. Description of the Background Art
FIG. 1 is a block diagram illustrating an interior of a conventional video camera. As shown therein, the conventional video camera includes a charge coupled device (hereinafter, referred to as CCD) 100 for converting, to an electrical signal, the optical image received through a lens and outputting a resultant value, a correlated double sampling (hereinafter, referred to as CDS) and automatic gain control circuit (hereinafter, referred to as AGC) circuit 110 for carrying out sample/hold and automatic gain control with regard to the output signal of CCD 100, a reference voltage generator 130 for generating an upper phase reference voltage VT and a lower phase reference voltage VB, an analog/digital converter (hereinafter, referred to as A/D converter) 120 for converting the analog image signal outputted from the CDS/AGC 110 to a digital image signal in accordance with reference signals outputted from the reference voltage generator 130, and a clamp circuit 140 for clamping a black level value of the analog image signal outputted from the CDS/AGC circuit 110 to the lower phase reference voltage value VB outputted from the reference voltage generator 130 so that these values become equal and feeding back the clamped value to the CDS/AGC circuit 110, and a potentiometer R for varying the lower phase reference value VB received by the A/D converter 120.
The black level calibrating process of a video camera according to the conventional art will now be described.
First, the output signal of the CCD 100 realizes its sample/hold and automatic gain control through the CDS/AGC 110, and is converted to a normal analog image signal. The analog image signal is converted to a digital image signal by the A/D converter 120.
The A/D converter 120 converts the analog image signal to a digital image signal with reference to the upper phase reference voltage VT and the lower phase reference voltage VB, as varied by a potentiometer R, generated by the reference voltage generator 130 and outputs a resultant value.
At this time, since the black level value of the applied analog image signal has to correspond to the lower phase reference voltage value VB, there is provided a black level clamp circuit 140.
The black level clamp circuit 140 receives a black level signal periodically generated from the CDS/AGC circuit 110, clamps the received signal to a level of the lower phase reference voltage VB generated by the reference voltage generator 130 and feeds the result back to the CDS/AGC circuit 110.
Therefore, although the lower phase reference voltage VB of the A/D converter 120 and the black clamping level of the black level clamp circuit 140 are theoretically equal, it is difficult for an accurate clamping to be ideally carried out due to mismatch with a system or device. Also, there may occur a disadvantage in which a black level should be controlled depending upon the quality of a particular image requested by a customer.
In order to eliminate such a disadvantage, the conventional video camera, as shown in FIG. 1, provides the potentiometer R (i.e., variable resistance) attached to a lower phase reference voltage output terminal of the reference voltage generator 130 to appropriately and manually control the lower phase reference voltage VB received by the A/D converter 120. The value from the variable resistance R is applied to the A/D converter 120, whereby the lower phase reference voltage VB of the A/D converter 120 is controlled differently from the reference voltage applied to the black level clamp circuit 140.
Although the conventional circuit described above has not been problematic in the conventional system in which respective functions of CDS/AGC, A/D conversion and reference voltage generation are realized in separate chips by attaching a variable resistance outside the chip, a considerable disadvantage is incurred in the case where these function are placed on a single chip.
Further, since the reference voltage applied to the A/D converter 120 is varied, an internal conversion range of the A/D converter is varied, which results in deterioration of the A/D converter function.
In the black level calibration apparatus according to the present invention, a reference voltage generator generates a reference voltage, and a correlated double sampling/automatic gain control (CDS/AGS) circuit perform sample/hold and automatic gain control operations on an analog image signal based on a black level clamping signal. The apparatus also includes a black level clamping circuit which generates the black level clamping signal based on a calibrated reference voltage. An analog to digital converter converts the analog image signal output from the CDS/AGS circuit into a digital image signal. A control signal setting circuit compares the black level of the digital image signal with a black level calibration value, and sets a digital control signal based on the comparison. A reference voltage calibration circuit generates the calibrated reference voltage in accordance with the digital control signal. Because these circuits are included on a single chip and do not require an external variable resistance, the problems and disadvantages discussed above with respect to the conventional art are overcome.
The object and advantages of the present invention will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific example, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.